Actually, the cartridge dimensions for the 5.56x45mm and .223rem are not identical. Specifically, the 5.56x45mm has a thicker case, therefore less powder capacity, and the 5.56 also is rated to higher chamber pressures than the .223rem. Additionally, the chamber throat is longer for the 5.56. So in a .223rem the longer 5.56mm cartridges may actually seat the bullet into the rifling when chambered, which can create a dangerous pressure spike.

So typically, since the 5.56mm NATO is the LONGER and HIGHER PRESSURE of the two, it is safe to fire .223rem from a 5.56mm chambered rifle, but it is not recommended to fire 5.56 NATO rounds from a .223 chamber. However, from my personal experience, the excessive throat jump when firing shorter .223 rem cartridges in 5.56 NATO chambers has a negative effect on accuracy. For the handloader, the .223rem brass can be loaded with the bullets slightly longer in the case to reduce the throat jump and improve accuracy.

THAT SAID, in modern firearms many manufacturers have recognized the danger of consumers mixing up cartridges, and therefore are proofing their .223rem chambers to withstand 5.56 NATO pressures. HOWEVER, the chamber throats are still machined shorter, so excessive pressure from seating the bullet into the rifling still represents danger. Shooters can mark their bullets with magic markers to test to see if the bullet is contacting the rifling when chambered. If so, then the 5.56NATO cartridges should not be used in their rifle, if not, the shooter should set the bullet slightly longer, and test until the COL to reach the rifling is found to determine if the throat jump is safe.

Alternatively, the cartridge dimensions for the 7.62NATO and .308win are minimal with the major difference coming in the pressure rating. However, SAAMI has determined that the two cartridges can be used interchangeably. The real danger is for the handloader who might mistakenly charge a 7.62 NATO case (lower capacity) with a full .308win powder charge, creating excessive chamber pressures.

Based on SAAMI pressure limitations, the .308win should NOT be fired in older 7.62NATO rifles. The .308win max pressure is 24% higher than the 7.62 NATO. However, in modern rifles, the same barrel blanks are used for both rifles, with the same dimensions of steel surrounding each chamber. The steel can withstand the same pressure regarless of the headstamp on the brass, so modern rifles won't notice any difference between the two.